Many RF communications systems have RF receivers that need to receive a desired RF signal on a specific RF channel, which is a desired RF channel that has a desired bandwidth and a desired RF center frequency. One function of the RF receiver is to reject any RF signals at frequencies other than those within the desired bandwidth of the desired RF channel; therefore, numerous filtering and signal rejection techniques have been developed to achieve this function. One such technique is called super-heterodyning, in which received RF signals are filtered and then mixed with a local oscillator signal to down convert the filtered RF signals into lower frequency signals, which are known as intermediate frequency (IF) signals. The mixing down converts a desired RF signal into a desired IF signal having a desired IF center frequency. Generally, it is easier to filter out unwanted signals that are close in frequency to desired signals at IF frequencies than it is to filter the same signals at higher RF frequencies. However, mixers have a characteristic that produces image signals in addition to desired signals. Image signals may be removed by RF filtering, IF filtering, or both.
In any heterodyne receiver, when a received RF input signal FR mixes with a local oscillator signal FLO, the mixer produces an output signal with sums and differences of FR and FLO. Specifically, the frequencies of FR−FLO and FR+FLO, or FLO−FR and FR+FLO, are the dominant mixer output frequency combinations. If FLO is chosen with a lower frequency than a desired RF input signal FDRF, then an FR−FLO portion of the mixer output signal produces a desired IF signal FDIF; however, the mixer output signal will also include an FR+FLO image signal, which is close to double the frequency of FDRF and easily removed by IF filtering. If a blocking image signal FBIS with a frequency located at a frequency of FLO minus the frequency of FDIF is received, the FR−FLO portion of the mixer output will produce an image that is identical in frequency to FDIF but phase-shifted 180 degrees from FDIF, and cannot be removed with normal IF filtering techniques; therefore, if upstream RF bandpass filtering cannot remove the blocking image signal, then other techniques must be used to remove the signal. However, since the blocking signal is phase-shifted by 180 degrees from FDIF, a quadrature receiver architecture can be used to filter out the blocking image signal. A quadrature receiver architecture uses two mixers receiving the same RF input signal, which is mixed with two different local oscillator signals that are equal in frequency and phase-shifted from each other by 90 degrees. Complex filtering methods can then be used to filter out the blocking image signal. Any mismatch between the processing of in-phase signals and quadrature-phase signals will result in degradation of the rejection of image signals.
For example, if the desired RF input signal FDRF is at 900 Mhz and the local oscillator signal FLO is at 899 Mhz, then the desired IF signal FDIF is at 1 Mhz (FR−FLO). Further, an IF image signal is at 1799 Mhz (FR+FLO), which is easily filtered out in the IF section. A blocking image signal FBIS at 898 Mhz will produce a blocking IF signal at −1 Mhz (FR−FLO), which is phase-shifted 180 degrees from FDIF. If the blocking image signal FBIS cannot be filtered out in the RF section, then complex filtering methods can be used to filter out the blocking image signal in the IF section.
Some RF communications protocols include as many channels as possible in a given bandwidth; therefore, channel spacing may be tight. As a result, desired IF center frequencies may be reduced to maximize adjacent channel and alternate channel rejection. Some communications systems use very low intermediate frequencies (VLIF) or even down convert such that the desired IF center frequency is zero, which is known as a direct conversion receiver (DCR); however, lower IF center frequencies tend to produce certain side effects. With a DCR, 1/f noise increases, direct current (DC) offsets, and second-order inter-modulation (IIP2) effects may be difficult to remove. As a result, effective receiver sensitivity may be reduced. The optimum desired IF center frequency may vary depending on the signal strengths of desired channels, adjacent channels, and alternate channels.
In some networks, there is a loose correlation between the signal strength of a desired signal and the signal strength of interfering image signals; therefore, when the signal strength of a desired signal is small, a higher desired VLIF frequency is desirable to increase receiver sensitivity. The resulting reduced image rejection is acceptable, since the signal strengths of interfering image signals are also small. Likewise, when signal strengths of interfering image signals are large, a lower desired VLIF frequency is desirable to increase image rejection. The resulting reduced receiver sensitivity is acceptable, since the signal strength of the desired signal is also large; therefore, in some networks, it would be beneficial to have an inverse correlation of the desired VLIF center frequency with signal strength.
Given the above factors, there is a need for an RF receiver that can change its desired IF center frequency based on signal strengths of received signals. Additionally, there is a need for a quadrature RF receiver with complex filtering having matched circuitry processing the in-phase signals and the quadrature-phase signals to eliminate blocking image signals that cannot be filtered out in the RF section of the quadrature RF receiver.